An acoustic sensor of a capacitance type has a structure in which a diaphragm (movable electrode plate) and a fixed electrode plate are provided on a top surface of a hollow (through hole) formed in a substrate. A microphone is obtained by placing an acoustic sensor and a process circuit on a bottom face in a package and forming package sound holes for introducing acoustic oscillation in the package. It is known that, to improve the acoustic characteristics such as sensitivity and frequency characteristic of such a microphone, the capacity of a space (called a back chamber) on the side opposite to the side where the acoustic oscillation enters using the diaphragm as a reference is increased.
In the microphone, generally, package sound holes are formed in the top surface of the package. In a microphone of this type, acoustic oscillation which passes through the package sound holes and enters the package passes through the fixed electrode plate and the diaphragm and enters the hollow. At that time, the acoustic oscillation oscillates the diaphragm to change a capacitance value between the diaphragm and the fixed electrode plate. Therefore, in the microphone, since the hollow in the substrate becomes the back chamber, the capacity of the back chamber cannot be increased so much.
As a practical method for improving acoustic characteristics such as the sensitivity and the frequency characteristic of a microphone, a method of opening a package sound hole in a package in a position where the hole is directly connected to a hollow in a substrate, that is, just below the hollow is proposed (refer to FIG. 1A).
As another method for improving acoustic characteristics such as the S/N ratio (signal-to-noise ratio) and sound pressure band of a microphone, there is a method of providing two acoustic sensors in a microphone. When two acoustic sensors are provided in a single package, by adding outputs of the two acoustic sensors, the sensitivity of the microphone can be improved and noise cancelling can be performed. As a result, the S/N ratio can be improved. By internally providing two acoustic sensors having different sensitivities, different sound pressure bands, different frequency bands, and the like, by using both outputs of the acoustic sensors while switching them in circuits on the post stage, characteristics which cannot be realized by single acoustic sensor can be obtained. For example, by using both an acoustic sensor having high sensitivity and adapted to low sound pressure and an acoustic sensor having low sensitivity and adapted to high sound pressure and switching the acoustic sensors according to the sound pressure bands, a microphone of a wide band having high sensitivity and adapted to high sound pressure can be realized artificially.
As a microphone incorporating a plurality of acoustic sensors, for example, there is a microphone disclosed in U.S. Unexamined Patent Application Publication No. 2007-47746. In the microphone disclosed in U.S. Unexamined Patent Application Publication No. 2007-47746 (FIG. 3A), however, since a plurality of acoustic sensors are disposed on the bottom face of a package and the package sound holes are open in the top surface of the package, the package sound holes cannot be directly connected to the hollows in the acoustic sensors.
As an example of improving the microphone disclosed in U.S. Unexamined Patent Application Publication No. 2007-47746 (FIG. 3A), as illustrated in FIG. 1, a plurality of acoustic sensors 13a, 13b, . . . independent of one another are mounted on the upper face of the bottom of a package 12 and package sound holes 14 directly connected to hollows 17 are provided in the bottom of the package 12. Since this microphone 11 includes a diaphragm 15 and a fixed electrode plate 16 on the top surface of each of the acoustic sensors 13a, 13b, . . . , the hollow 17 in the acoustic sensor becomes a front chamber, and a package space 18 in the package becomes a back chamber. Therefore, the capacity of the back chamber can be increased, and the characteristics of the microphone can be improved.
In a microphone of such a structure, however, since a package sound hole is provided for each acoustic sensor, there is the possibility that the acoustic sensors detect acoustic oscillations which enters from the different package sound holes and are slightly different from one another. When output signals of the detected acoustic oscillations which are slightly different from one another are added as described above, for example, there is fear that the output signals interfere one another and buzz occurs. When a plurality of independent acoustic sensors are used as illustrated in FIG. 1, there is the case that manufacture variations among the acoustic sensors become an issue.
On the other hand, in the acoustic sensor disclosed in U.S. Unexamined Patent Application Publication No. 2007-47746 (FIG. 4), as illustrated in FIG. 2, the fixed electrode plate 16 is provided on the top surface of a substrate 22, a plurality of diaphragms 15 are provided above the fixed electrode plate 16, and a plurality of sensing elements 21a, 21b, . . . (capacitor structure) are formed by the diaphragms 15 and the fixed electrode plate 16. In each of the sensing elements 21a, 21b, acoustic holes 23 are open in the fixed electrode plate 16. In the acoustic sensor 13 as illustrated in FIG. 2, the plurality of sensing elements 21a, 21b, . . . are formed in the single substrate, so that manufacture variations of the sensing elements are small. Therefore, it is considered to form one package sound hole in the package so as to be directly connected to the under surface of the hollow 17 by using the acoustic sensor 13 as illustrated in FIG. 2.
In the acoustic sensor 13 of FIG. 2, as it is convenient that the sensing elements 21a, 21b, . . . share the hollow 17, the hollow 17 extends in the entire space below the sensing elements 21a, 21b, . . . . On the other hand, in the hollow 17, a reinforcing member (stiffening rib) 24 is provided by the substrate 22 in an upper part of the hollow 17.
However, since the stiffening rib 24 is an etching residual at the time of forming the hollow 17 in the substrate 22 by etching and is a member which is much thinner than the substrate 22, sufficient strength cannot be given to the acoustic sensor 13 by the stiffening rib 24 itself. Consequently, the substrate 22 is distorted by an impact given when the microphone is dropped, and the diaphragm 15 is easily broken.
In the acoustic sensor 13 of FIG. 2, the etching volume at the time of forming the hollow 17 in the substrate 22 is large, so that the etching time is long, and the productivity of the acoustic sensor is low. Further, in the acoustic sensor 13, the hollows 17 below the sensing elements 21a, 21b, . . . are connected. Consequently, the acoustic oscillation which enters the hollows 17 easily escapes from the entire sensing element, and the low-frequency characteristic of the acoustic sensor 13 deteriorates.
In the acoustic sensor 13 of FIG. 2, the position of providing the package sound hole is limited to the opening area in the under surface of the hollow 17, so that the freedom degree for designing the position of the package sound hole is low, and a foreign matter such as dust easily enters the hollow 17 from the package sound hole.
In the acoustic sensor of U.S. Unexamined Patent Application Publication No. 2007-47746 (FIG. 4), a partition wall 25 is constructed by extending the stiffening rib 24 to the under surface of the substrate 22, and the hollows 17 can be partitioned by the partition wall 25. By forming a communication hole 26 at a height in a center part of the partition wall 25, the neighboring hollows 17 are communicated (indicated by a broken line in FIG. 2). However, in such a modification, the communication hole 26 has to be formed so as to laterally penetrate the partition wall 25 in the center part of the partition wall 25, so that the process of opening the communication hole 26 is extremely difficult. Further, when the partition wall 25 is provided but the communication hole 26 is not provided, a package sound hole has to be formed for each of the hollows 17, and an inconvenience similar to that of the case of FIG. 1 occurs.